S a

Table 9. Size and Cost Comparison: Spray Dryer vs. Spin Flash Dryer

Spray dryer type III-K Spin flash type III

Space requirements no. 70/71 no. 59

Space requirements

Heater type (gas) Direct-fired Direct-fired

Chamber diameter, ft 14'0" 2'7-l/2"

Building floor area, ft2 650 325

Building height, ft 46'0" 16'6"

Building volume, ft3 24,700 5,300


Powder, lb/h 880 880

Performance data

Feed solids, % 30 45

Feed rate, lb/h 2,995 1,951

Water evaporation, lb/h 2,115 1,071

Gas consumption, SCF/h" (1000 Btu/SCF) 4,416 2,192

Power consumption (kWh) 40 30

Building 75,000 20,000

Dryer equipment 174,000 147,000

Baghouse 22,000 18,000

Installation 35,000 22,000

$306,000 $207,000


Gas cost, $/M Btu $4.32 Electricity cost, $/kWh $0.07 Salaries per hour $15.00 Manpower required = 1 operator

Variable operating costs per hour

Gas 19.08 9.47

Electricity 2.80 2.10

Note: It can be seen that the investment in a spin flash dryer plant is about 32% lower than a spray dryer for the same capacity and that spin flash operating costs are approximately 29% lower. These figures, however, do not include the capital investment in filtration equipment. "Standard cubic feet.

The drying chamber would be maintained at a pressure slightly higher than ambient using a pressurized nitrogen purge (17). The heater (5) would use either a steam coil or a thermal fluid system with an external heater.

A less expensive alternative to the closed-cycle approach is a "Lo-Ox" (low-oxygen) system where a low excess air burner is used in a direct-fired heater. The products of combustion are recirculated through the condenser, and the surplus gas is vented to atmosphere. The oxygen level in such a system can be controlled to less than 5%. (See also Figure 41.)

Spin-Flash-Dried Products

While Table 8 charts some typical products being dried on spin flash equipment, it should be noted that in some cases the inlet temperatures are limited by the heat source used and not by the spin flash process.

Other products test dried on a laboratory scale spin flash dryer include the following:

Feed Solids, Powder Moisture, Product % %

Iron oxide 70 2

Effluent treatment sludge 22 40

Lignin 49 15

Gum 40 13

Chitosan gel 6 40

Crab meat paste 30 9

Dutched cocoa cake 70 2

Cost Benefits

In comparison to a spray dryer, the spin flash dryer has a much shorter residence time and consequently is considerably smaller and requires less building space. Its ability to dry to even higher solids than a spray dryer results in operating cost savings. Table 9 shows a detailed size and cost comparison based on actual test drying of yellow iron oxide.


Despite its obvious size and cost advantages, there are many instances when a spin flash dryer cannot replace a spray dryer. Typically, such cases occur when a free-flowing spherical particle of a particular size range is required or when agglomeration is needed.

There are, however, many situations in both the food and chemical industries where the particular capabilities of the spin flash dryer to produce powders from paste warrant careful consideration of its use.

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